Flash memory devices are Electrically Erasable Programmable Read Only Memory (EEPROM). Flash memories may be widely employed, for example, in computers, digital cameras, MP3 players, game systems, memory sticks, and so on. The flash memory devices electrically control data to be input or output in the mechanism of Fowler-Nordheim (FN) tunneling effect or hot electron injection.
In view of electric circuit, the flash memory devices may be generally classified into a NAND type where N-numbered cell transistors are serially connected each other to form a unit string and a plurality of unit strings is connected in parallel between a bit line and a ground line, and a NOR type where cell transistors are serially connected between a bit line and a ground line.
Such a flash memory device must be designed to store a large capacity of data as much as possible. Therefore, a unit chip is required to include a multiplicity of cell transistors. However, there is a limit in highly raising integration density of cell transistors within a defined horizontal area.
A three dimensional memory structure may be helpful to overcome the two dimensional scaling limit of the NAND flash memory. Structures of three dimensional NAND flash memories may include Built-In Current Sensor (BICS) types and piped Bit Cost Scalable (BiCS) types which are advanced from the BIOS types.